CODEWITHSUNDEEP
pythondictionary
sheth7
sheth7

Reputation: 349

conditionally adding dictionary entries from a second dictionary to a new dictionary while preserving new dictionary keys

I have two dicts- dict1 = {'a':'value','b':'value','d':'value'} and dict2 = {'p':'blank','q':'blank','r':'blank','s':'blank'}. Their keys are stored in two lists- list1 = ['a','b','c','d'] and list2 = ['p','q','r','s']. dict1 might not always contain all key:value pairs.

I want to add entries from dict1 to dict2, if keys exist in dict1 else I want "blank"

I wrote this code:

for i in list1:
    if i in dict1.keys():
        dict2[i] = dict1[i]
    else:
        dict2[i] = "blank"

This will give me dict2 with keys from list1. How can I modify this code to preserve the conditionality but keep keys associated with dict2 i.e., ['p','q','r','s']? My desired output

dict2 = {'p':1,'q':2,'r':'blank','s':4}

The number of keys will always be the same in the two dicts.

Also, is there a more pythonic way to do this? I saw some answers here but did not understand completely.

Upvotes: 1

Views: 63

Answers (2)

jignatius
jignatius

Reputation: 6494

If I understand you correctly, you want to update dict2 if the item in list1 is in dict1 using the corresponding key in list2.

You can zip the lists and iterate over the keys in both lists, setting the value using a conditional statement.

dict1 = {'a':1,'b':2,'d':4}
dict2 = {'p':'blank','q':'blank','r':'blank','s':'blank'}
list1 = ['a','b','c','d']
list2 = ['p','q','r','s']

for a, b in zip(list1, list2):
    dict2[b] = dict1[a] if a in dict1 else 'blank'

print(dict2)

Output:

{'p': 1, 'q': 2, 'r': 'blank', 's': 4}

Alternatively you can use the dictionary's update() method and update all the values in one line:

dict2.update({b : dict1[a] if a in dict1 else 'blank' for a,b in zip(list1, list2)})

update() takes a dictionary of items. In the line above you are creating a temporary dictionary with a dictionary comprehension and updating dict2.

Upvotes: 0

Aurel Bílý
Aurel Bílý

Reputation: 7983

So if I understand correctly, you want to "map" the keys of list1 to the keys of list2? In which case, you can zip the two lists to iterate over the two lists in order, then keep the rest of the code same. One small improvement is that i in dict1.keys() is actually the same as i in dict1.

for i, j in zip(list1, list2):
    if i in dict1:
        dict2[j] = dict1[i]
    else:
        dict2[j] = "blank"

Upvotes: 1

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